Treatment of heat transfer salts



Patented May 15, 1945 TREATMENT OF HEAT TRANSFER SALTS Vladimir A.Kalichevsky, Frederick E. Bay, and John W. Payne, Woodbury, N. J.,assignors to Socony-Vacuum Oil Company, Incorporated, New York, N. Y., acorporation of New York No Drawing. Application September 28, 1940,

Serial No. 358,828

2 Claims.

This invention relates to methods of maintaining the desirableproperties of heat transfer salts, with special reference to alkalimetal salts of the oxyacids of nitrogen.

Heat transfer salts with as low a melting point as possible aredesirable for controlling temperatures in chemical and other equipment.Thus. where the temperature is maintained between say 700 F. and 900 F.in the zone controlled by the salt, it is desirable for the salt to havea substantially lower melting point, whereby it may be melted by thesteam available, which ordinarily runs 400 F. or lower. Thus when aplant is shut down and cooled it is desirable to be able to melt thesalt in the lines with such steam in tracers. We have found that in usethe properties of the salts change. resulting in an increased meltingpoint, in the formation of deposits which in some instances partiallyplug some of the lines and in all cases effect a reduction of the heatefliciency of the systems and in the deposition of solid phase from thesalt compositions. Accordingly, it is an object of this invention toprovide processes for the treatment of such heat transfer salts tomaintain a desired melting point thereof and to prevent the depositionof scale or other solid phase from the molten salt.

It has now been found that in the maintenance of heat transfer salts ofthe type described it is desirable to maintain the concentration ofalkali in the salt low, in general below about 5%, and preferablybetween about 0.2% and about 2%. This has been found to be ofsubstantial importance in maintaining a low melting point and inpreventing scale and other depositions, despite the fact that it hasbeen found that limited amounts of alkali reduce the melting point ofthe compositions.

In accordance with this invention, a molten salt composition comprisingprincipally alkali metal nitrate and nitrite is withdrawn from 'a zonein which it is employed in molten state and is treated with nitric acidto effect a reduction or lowering of the alkali content of the salt, andthe salt so treated is returned to the zone.

The salt composition is employed in the molten state in a zone which maybe a cyclic circulating system including in the circuit apparatus suchas a catalyst case, the temperature of which it is desired to control, aheat exchanger for adjusting the temperature of the composition, a pumpfor circulating the salt in the system and a surge tank for allowingsome change in the total volume 01' salt in the system. Suitable pipingis provided for conveying the salt from each unit to the next in thesystem. It should be understood that this system is merelyrepresentative of one form of system in which the molten salt may beemployed, and that the present invention, which relates to treatment ofsuch salts, is not limited to any particular type of system in which themolten salt is employed.

Salt is Withdrawn from the zone of use and is treated in a reducing zonein which the alkali content of the salt is reduced or lowered and thesalt so treated is returned to the zone of use. The salt is withdrawnfrom the zone of use, treated and returned at such a rate that thealkali in the zone is maintained below 5% and preferably between about0.2% and about 2%. In some instances the reduction may be carried to thecomplete elimination of the alkali in the reducer. By the term alkalithroughout this description and in the subjoined claims it is intendedto include both free alkali (alkali metal oxide or hydroxides) andalkali metal carbonates.

About 1,000,000 pounds of a molten salt containing 40% NaNOz, 7% NaNOsand 53% KNOa -was charged to a heat transfer system involving aplurality of catalytic converter cases, a heat exchanger and suitablepumps and surge tanks. After several months of use the alkali contentbuilt up to 1.43% (calculated as NaOH). Salt was withdrawn from thesystem and dissolved in water at a rate of 10 gallons of water perpounds of salt. Normal nitric acid was added at a rate of 4 gallons per100 pounds of salt. The treated salt was then dried and melted. The saltso treated while having a melting point slightly higher than prior totreating does not tend to form scale and may be employed by addition tothe system to control scaling therein.

While within the broad scope of this invention the salt may be treatedin any state with nitric acid, in accord with a preferred form hereofthe salt is dissolved in water and the solution is treated with thenitric acid. This procedure is especially desirable due to theconservation of fixed nitrogen and to the completeness of the removal ofalkali which may hereby be effected.

'We claim as our invention:

1. The process of treating an alkali metal nitrate and nitrite saltcomposition which in use is maintained in circulation in a heat transfersystem in molten state, comprising withdrawing salt from said heattransfer system after it has been used therein with resulting formationof free alkali, treating the withdrawn salt with nitric acid to effect alowering of the free alkali content ofthe salt by neutralization,returning salt so treated to said heat transfer mum, and eii'ectin: thewithdrawing. treating and returning at a rate to maintain the alkalicontent in eaid'heat transfer system while circulating between 0.2% and2%.

2. In a process of treating a salt mixture consisting principally ofalkali metal nitrate and nitrite which has been maintained incirculation in a molten state in an indirect heat transfer system and inwhich alkalinity and melting point have increased as aresult of suchuse. the steps comprising contacting said molten mixture with nitricacid until the alkalinity and melting point have been decreased, andreturning the molten mixture for indirect heat transfer.

VLADIMIR. A. KALICHEVBKY.

FREDERICK E. RAY.

JOHN W. PAYNE.

